It is well known that ultraviolet light having a wavelength between about 280 nm or 290 nm and 320 nm (UV-B) is harmful to human skin, causing burns that are detrimental to the development of a good sun tan. UV-A radiation, while producing tanning of the skin, also can cause damage, particularly to very lightly colored, sensitive skin, leading to reduction of skin elasticity and wrinkles. Therefore, a sunscreen composition should include both UV-A and UV-B filters to prevent most of the sunlight within the full range of about 280 nm to about 400 nm from damaging human skin.
The UV-B filters that are most widely used commercially in sunscreen compositions are paramethoxycinnamic acid esters, such as 2-ethylhexyl paramethoxycinnamate, commonly referred to as octyl methoxycinnamate (e.g., PARSOL MCX), octyl salicylate; homomenthyl salicylate, and others.
The UV-A filters most commonly used in commercial sunscreen compositions are the dibenzoylmethane derivatives, particularly butyl methoxybenzoylmethane (avobenzone), 4-isopropyl dibenzoylmethane (EUSOLEX 8020), and other dibenzoylmethane derivatives described in U.S. Pat. Nos. 4,489,057; 4,387,089 and 4,562,067, hereby incorporated by reference. It is also well known that some of the UV-A filters, or combinations of UV-A and UV-B filters, suffer in photochemical stability (see R. M. Sayre, J. C. Dowdy, Photostability Testing of Avobenzone, Cosmetics & Toiletries Magazine 114, 85-91 (May, 1999); and N. Tarras-Wahlberg et al., Changes in Ultraviolet Absorption of Sunscreens After Ultraviolet Irradiation, J. Invest. Derm. 113, 547-553 (1999)). Accordingly, when a UV-B filter, such as 2-ethylhexyl-p-methoxycinnamate (PARSOL MCX), is combined with a dibenzoylmethane derivative UV-A compound, such as PARSOL 1789, the combination becomes less photochemically stable necessitating repeated, frequent coatings over the skin for sufficient UV radiation protection (see Bonda, et al. The Photochemistry of Sunscreen Photostability, page 5).
In accordance with the principles of the present invention, it has been found, quite surprisingly, that by including a barium compound, such as barium sulfate, into a sunscreen formulation containing a combination of a UV-A dibenzoylmethane derivative, particularly avobenzone, and a methoxycinnamate derivative UV-B sunscreen agent, such as octyl methoxycinnamate, the sunscreen combination is photochemically stabilized so that the sunscreen composition is more effective for filtering out UV-A and UV-B radiation; the composition filters both UV-A and UV-B radiation for longer periods of time; and, therefore, the sunscreen formulation need not be applied to the skin as frequently while maintaining effective skin protection against UV-A and UV-B radiation.
By the addition of the barium compound, the cosmetic sunscreen formulation can maintain surprisingly effective skin protection against UV radiation both in the UV-A and UV-B range, with or without additional common sunscreen additives, such as benzophenone 3, octocrylene, and/or titanium dioxide. Preferably, the ratio of UV-A to UV-B filter compounds is in the range of about 0.1:1 to about 3:1, preferably about 0.1:1 to about 0.3:1, most preferably about 0.24:1.